Paramètres de conception optimaux pour maximiser le rapport contraste à bruit pour scanners TEP avec temps de vol

Abstract : Time-of-flight (TOF) positron emission tomography (PET) scanners improve contrast-tonoise ratio (CNR) that translates into reducing the scan time or the required injected dose. During the past years, TOF PET has evolved towards temporal resolutions of the order of 200 ps, corresponding to a spatial uncertainty of 30 mm along the line of response (LOR) defined by the two annihilation photons. Although this location uncertainty is sufficient to improve the effective sensitivity of clinical scanners, resolving small size tissues such as a lymph node, or small animal organs would require the timing performance to be less than 50 ps to resolve objects smaller than ⇠ 10 mm. A coincidence time resolution around 10 ps would even allow to avoid tomographic reconstruction of PET images. Obtaining good image performance in PET demands tackling simultaneously all image quality parameters, including spatial resolution, sensitivity, and CNR. However, this involves difficult trade-offs as studies have demonstrated that choices made at the design level for the detector configuration may enhance some image quality parameters but are then detrimental to others. It is therefore mandatory to identify and carefully investigate the factors contributing to the CNR, one of the most important parameter for image quality. One such factor is the choice of crystal thickness that affects coincidence time resolution and thus CNR. Although improved coincidence time resolution increases the chance of small lesion detectability, trade-offs should be studied to find an optimum compromise maximizing the image performance. The motivation underlying this research is to determine the limit where TOF adds gain in small animal PET imaging and also investigate trade-offs between crystal length, timing resolution, and sensitivity to find the optimum image quality. These trade-offs target the coincidence time resolution improvement to enhance CNR performance without compromising the other parameters of image quality. It is demonstrated that a coincidence time resolution of 100 ps is the threshold where TOF starts to improve the image performance of a small animal scanner. In addition, it is shown that the crystal thickness can be reduced by 19 % without loss on the imaging performance. A model is also proposed that describes the CNR performance with a relatively high level of confidence at early stages of the design, and can be used as a guide to design the future generation of scanners. This is followed by introducing a new phantom purposely designed to study TOF benefits and impacts on lesion detectability for PET scanners.Les scanners de tomographie d’émission par positrons (TEP) par temps de vol (TdV) augmentent le rapport contraste à bruit (RCB) en réduisant le bruit de fond. Ceci se traduit par un temps d’acquisition plus court ou une dose réduite. Au cours des années, la TEP-TdV a évolué vers des résolutions temporelles de l’ordre de 200 ps, ce qui correspond à une incertitude spatiale de 30 mm. Bien que cela soit suffisant pour améliorer la sensibilité effective des scanners cliniques, résoudre des petites structures comme les ganglions lymphatiques, ou des organes de petits animaux nécessite des résolutions temporelles inférieures à 50 ps pour résoudre un objet inférieur à ∼ 10 mm. Une résolution temporelle de 10 mm permettrait même d’éviter la reconstruction tomographique des images TEP. L’obtention d’une bonne performance d’image en TEP nécessite d’aborder simultanément tous les paramètres de qualité d’image, y compris la résolution spatiale, la sensibilité et le RCB. Cependant, il est peu probable que cela se produise, car des études ont démontré que les choix de conception du détecteur peuvent favoriser certains paramètres de qualité d’image, mais en dégrader d’autres. On doit donc cibler les facteurs contribuant au RCB, l’un des paramètres importants de la qualité d’image. Un de ces facteurs est le choix de l’épaisseur du cristal qui affecte la résolution temporelle et donc, le RCB. Bien qu’une résolution temporelle améliorée augmente la détectabilité des petites lésions, on doit étudier les compromis afin de trouver un point d’équilibre offrant à la meilleure performance d’image possible. La motivation de cette recherche est de déterminer la limite à partir de laquelle le TdV améliore la qualité de l’imagerie des petits animaux et également, d’étudier les compromis nécessaires entre la longueur des cristaux, la résolution temporelle et la sensibilité pour atteindre la qualité d’image optimale. Ces compromis ciblent l’amélioration de la résolution temporelle pour améliorer les performances du RCB sans compromettre les autres paramètres de qualité d’image. Ces travaux démontrent qu’une résolution temporelle de 100 ps est le seuil à partir duquel le TdV améliore le performance RBC de l’imagerie des petits animaux. De plus, ils montrent que le volume du cristal peut être réduit de 19 % sans détériorer l’image. Un modèle est également proposé pour prédire le RCB avec un niveau de confiance relativement élevé et il peut être utilisé comme guide pour concevoir la prochaine génération de scanners. L’introduction d’une nouvelle mire élaborée pour étudier les avantages et les impacts du TdV sur la détectabilité des lésions pour les scanners TdV est par la suite présentée

Design and Characterisation of an MRI Compatible Human Brain PET Insert by Means of Simulation and Experimental Studies

Positron emission tomography (PET) is a widely used in-vivo imaging technique to visualise metabolism, allowing for a broad spectrum of applications in oncology, cardiology and neuroscience. At present, an MRI compatible human brain PET scanner for applications in neuroscience is being constructed in the scope of a Helmholtz Validation Fund project. In this thesis, a detector for this novel PET device was designed. The detector concept combined three scintillator layers with a lightguide and digital silicon photomultipliers (dSiPMs). Monte Carlo simulations were used to optimise the dimensions of the scintillator arrays, so that the new scanner design yielded the maximum possible sensitivity. The benefit from the additional depth information, which can be acquired with three scintillator layers, was evaluated and proven to be higher compared to a less expensive two layer geometry. Since a more homogeneous spatial resolution was achieved in the whole field of view, this finding had a high relevance for the envisaged neuroscientific applications. In order to accurately acquire the depth information, new strategies for decoding the flood map during the calibration of a detector module were developed. This required realistic simulation data with ground truth information, so that the simulation toolkit GATE was extended to model the electronic readout of the dSiPMs. To overcome extended simulation times and to provide simulations on a statistically sound basis, the GATE studies were executed on the supercomputer JURECA. The simulated data were matched to measured data from test detectors. This allowed the determination of an optimum thickness of a lightguide between the scintillators and the dSiPMs. Moreover, the number of correctly identified scintillation events was evaluated by means of different event positioning approaches and different clustering methods during the calibration step. The highest amount of correctly identified events in a single detector block was achieved with model-based clustering and Maximum Likelihood positioning (61.5 %). By simulating the whole propagation and detection of scintillation photons including ground truth information, this study provides the opportunity to improve the positioning approaches and to enhance this number in future. The gained insights were further applied to select a surface finish of the scintillators. Measurements with crystal samples of the final detector dimensions showed that rough lateral crystal surfaces yielded the best signal separation in the calibration flood map. The experimental and simulation studies presented in this thesis had a major influence on the final detector design of the novel brain PET. The detailed simulations including the propagation and detection of scintillation photons were in good agreement with measured data, and could be a promising approach for future detector design studies